The last 10 foot section of the equine intestine is the small colon. One of it's main roles is to create fecal balls, and remove water from them in order to conserve water. There is a wide range of manure consistency (from soft to firm) that is considered normal, and the occasional production of somewhat dry manure is probably not a cause for concern.

That said, very hard and dry manure can be a sign of dehydration or illness. For manure to become hard and dry, the large and small colon must have extracted excessive water from it. This happens when a horse is poorly hydrated.

Similarly, in a horse suffering from an underlying illness, manure may sit in the colon for a longer-than-normal period of time. Water is extracted by the colon during this time and the manure is dryer once expelled. Harder, drier manure is also more commonly seen in the winter, when water consumption decreases.